Setting up a "Favorites"/Bookmark function using Airtable

I'm trying to create an API patch request to allow users to add books to their favorites.

I have stored the books and account data in AirTable, however I can't figure out if it's possible to use JSON in the patch request to update a linked record (for example a record that links userID's to the corresponding book).

If it's not possible to patch a linked record, I could use AirTable Automations in combination with a non-linked field, but so far I've been able to set up a JSON script to patch the userID to a non-linked field, but once it runs it deletes the existing data. How would I go about patching a userID to the field, without removing the already present userID's.